For such a conventional optical disc device, a device as disclosed in Patent document 1 is known for example. Hereinafter, the conventional optical disc device will be described with reference to FIGS. 7-10.
FIG. 7(a) is a side view showing an optical disc device according to a conventional technique. FIG. 7(b) is a plan view showing a light source part of the optical disc device. FIG. 8 is a diagram showing a hologram pattern formed on a hologram used in the optical disc device. FIG. 9 is a diagram showing a photodetective pattern formed on a photodetector used in the optical disc device, and a light distribution of returning light on the photodetector, with respect to a first laser beam emitted from a first emission point of the light source. And FIG. 10 is a diagram showing a photodetective pattern formed on a photodetector used in the optical disc device, and a light distribution of returning light on the photodetector, with respect to a second laser beam emitted from a second emission point of the light source. Each of FIGS. 8-10 shows a hologram plane and a photodetective plane observed from the optical disc side.
As shown in FIGS. 7(a) and 7(b), the optical disc device according to the conventional technique includes a light source 1 such as a semiconductor laser, a collimator lens 4 for converting a light beam emitted from the light source 1 into parallel light, a quarter-wavelength plate 3 for converting linearly-polarized light into circularly-polarized light (or elliptically-polarized light) and converting circularly-polarized light (or elliptically-polarized light) into linearly-polarized light, an objective lens 5 for focusing the parallel light onto the optical disc, a hologram for diffracting the light (returning light) reflected by the optical disc, and a photodetector on which the returning light diffracted by the hologram is focused in a diffused state.
The light source 1 is attached to the photodetective substrate 12, having a first emission point 1a for emitting a first laser beam of a wavelength λ1 and a second emission point 1a′ for emitting a second laser beam of a wavelength λ2 (here, λ2>1). On the photodetective substrate 12, a reflecting mirror 10 is attached in the vicinity of the light source 1 in order to reflect laser beams emitted from the emission points 1a, 1a′, thereby bending the optical paths.
The hologram includes a polarization hologram substrate 13, and a hologram plane 13a formed on the polarization hologram substrate 13. The quarter-wavelength plate 3 is provided on the hologram plane 13a of the polarization hologram substrate 13 and configured to move integrally with the objective lens 5. As shown in FIG. 8, the hologram plane 13a is divided into four regions of a first quadrant, a second quadrant, a third quadrant and a fourth quadrant by two straight lines (x-axis, y-axis) crossing orthogonally at an intersection 100 between an optical axis 7 for the first laser beam and the hologram plane 13a. And furthermore, the respective quadrants are divided into strip-shaped regions along the x-axis: 91B, 91F; 92B, 92F; 93B, 93F; and 94B, 94F (hologram pattern).
The photodetector includes a photodetective substrate 12 and a photodetective plane 12a formed on the photodetective substrate 12. The photodetective plane 12a is located substantially at the focal plane position of the collimator lens 4 (that is, the position of a virtual emission point of the first emission point 1a shown in FIG. 7(b)). As shown in FIGS. 9 and 10, on the photodetective plane 12a, comb-tooth-shaped focus detection cells F1a, F2a, F1b, F2b, F1c, F2c, F1d, F2d, F1e and F2e are arranged along the Y-axis on the plus side of the Y-axis, and trapezoidal tracking detection cells 7T1, 7T2, 7T3, and 7T4 are arranged along the Y-axis on the minus side of the Y-axis (photodetective pattern), where the X-axis and Y-axis denote two straight lines crossing orthogonally at the intersection 90 between the optical axis 7 and the photodetective plane 12a, and parallel to the x-axis and y-axis respectively. These detection cells are arranged symmetrically with respect to the Y-axis. A light beam emitted from the first emission point 1a of the light source 1 travels in parallel to the X-axis in a plane that includes the X-axis and perpendicular to the sheet of drawing, and the light is reflected by the reflecting mirror 10 in the direction of the optical axis 7 (a direction passing the intersection 90 and orthogonal to the sheet of drawing).
As shown in FIGS. 7(a) and 7(b), the first laser beam (wavelength: λ1) emitted from the first emission point 1a of the light source 1 is reflected by the reflecting mirror 10, and focused by the collimator lens 4 so as to form a parallel light. This parallel light as linearly-polarized light (S-wave or P-wave) passes through the hologram and is converted into circularly-polarized light by the quarter-wavelength plate 3, then focused by the objective lens 5 so as to form a light spot on a signal plane 6a of a first optical disc 6. The light reflected by the signal plane 6a of the first optical disc 6 passes through the objective lens 5 and is converted into linearly-polarized light (P-wave or S-wave) by the quarter-wavelength plate 3, then enters a hologram plane 13a. The linearly-polarized light entering the hologram plane 13a is diffracted by the hologram plane 13a and split into first-order diffracted light 14 and minus first-order diffracted light 14′ with respect to the optical axis 7 serving as the symmetry axis. The respective diffracted light beams form convergent light beams through the collimator lens 4 and enter the photodetective plane 12a on the photodetective substrate 12. The diffraction efficiency of the returning light due to the hologram plane 13a is, for example, about 0% for a zeroth-order light, and about 41% for ±first-order light beams respectively.
The second laser beam (wavelength: λ2, where λ2>λ1) emitted from the second emission point 1a′ of the light source 1 is reflected by the reflecting mirror 10, focused by the collimator lens 4 so as to form a parallel light. This parallel light as linearly-polarized light (S-wave or P-wave) passes through (is partly diffracted by) the hologram and is converted into elliptically-polarized light by the quarter-wavelength plate 3, which then is focused by the objective lens 5 so as to form a light spot on a signal plane 6a′ of a second optical disc 6′. The light reflected by the signal plane 6a′ of the second optical disc 6′ passes through the objective lens 5 and is converted into linearly-polarized light (P-wave or S-wave) by the quarter-wavelength plate 3, and then enters a hologram plane 13a. The linearly-polarized light entering the hologram plane 13a is diffracted by the hologram plane 13a and split into a first-order diffracted light 15 and a minus first-order diffracted light 15′ with respect to the optical axis 7′ serving as the symmetry axis. The respective diffracted light beams pass through the collimator lens 4 so as to form convergent light, and enter the photodetective plane 12a on the photodetective substrate 12.
A light beam emitted from the first emission point 1a of the light source 1 and reflected by the signal plane 6a of the first optical disc 6 enters the hologram plane 13a. As shown in FIG. 9, first-order diffracted light beams 81B, 81F (not shown) diffracted by strip-shaped regions 91B, 91F on the first quadrant of the hologram plane 13a are focused on light spots 81BS, 81FS astride the border between detection cells F2a, F1b; minus first-order diffracted light beams 81B′, 81F′ (not shown) are focused on light spots 81BS′, 81FS′ formed within the detection cell 7T1. First-order diffracted light beams 82B, 82F (not shown) diffracted by strip-shaped regions 92B, 92F on the second quadrant of the hologram plane 2a are focused on light spots 82BS, 82FS astride the border between detection cells F1b, F2b; minus first-order diffracted light beams 82B′, 82F′ (not shown) are focused on light spots 82BS′, 82FS′ formed within the detection cell 7T2. First-order diffracted light beams 83B, 83F (not shown) diffracted by strip-shaped regions 93B, 93F on the third quadrant of the hologram plane 2a are focused on light spots 83BS, 83FS astride the border between detection cells F1d, F2d; minus first-order diffracted light beams 83B′, 83F′ (not shown) are focused on light spots 83BS′, 83FS′ formed within the detection cell 7T3. And, first-order diffracted light beams 84B, 84F (not shown) diffracted by strip-shaped regions 94B, 94F on the fourth quadrant of the hologram plane 2a are focused on light spots 84BS, 84FS astride the border between detection cells F2d, F1e; minus first-order diffracted light beams 84B′, 84F′ (not shown) are focused on light spots 84BS′, 84FS′ formed within the detection cell 7T4.
Since the first-order diffracted light beams 81B, 82B, 83B and 84B are focused on the back side of the photodetective plane 12a (i.e. on the further side from the hologram plane 13a), the light spots formed on the photodetective plane 12a are similar in form to the light distribution on the hologram plane 13a. Since the minus first-order diffracted light beams 81B′, 82B′, 83B′ and 84B′ are focused on the front side of the photodetective plane 12a (i.e. on the side nearer to the hologram plane 2a), the light spots formed on the photodetective plane 12a are similar in form to a light distribution obtained by inverting the light distribution on the hologram plane 13a with respect to a point 100. Since the first-order diffracted light beams 81F, 82F, 83F and 84F are focused on the front side of the photodetective plane 12a, the light spots formed on the photodetective plane 12a are similar in form to a light distribution obtained by inverting the light distribution on the hologram plane 13a with respect to the point 100. Moreover, since the minus first-order diffracted light beams 81F′, 82F′, 83F′ and 84F′ are focused on the back side of the photodetective plane 12a, the light spots formed on the photodetective plane 12a are similar in form to the light distribution on the hologram plane 13a. 
Similarly, a light beam emitted from the second emission point 1a′ of the light source 1 and reflected by the signal plane 6a′ of the second optical disc 6′ enters the hologram plane 13a. As shown in FIG. 10, first-order diffracted light beams 101B, 101F (not shown) diffracted by strip-shaped regions 91B, 91F on the first quadrant of the hologram plane 13a are focused on light spots 91BS, 91FS astride the border between detection cells F2a, F1b; minus first-order diffracted light beams 101B′, 101F′ (not shown) are focused on light spots 91BS′, 91FS′ formed within the detection cell 7T1. First-order diffracted light beams 102B, 102F (not shown) diffracted by strip-shaped regions 92B, 92F on the second quadrant of the hologram plane 13a are focused on light spots 92BS, 92FS astride the border between detection cells F1b, F2b; minus first-order diffracted light beams 102B′, 102F′ (not shown) are focused on light spots 92BS′, 92FS′ formed within the detection cell 7T2. First-order diffracted light beams 103B, 103F (not shown) diffracted by strip-shaped regions 93B, 93F on the third quadrant of the hologram plane 2a are focused on light spots 93BS, 93FS astride the border between detection cells F1d, F2d; minus first-order diffracted light beams 103B′, 103F′ (not shown) are focused on light spots 93BS′, 93FS′ formed within the detection cell 7T3. And, first-order diffracted light beams 104B, 104F (not shown) diffracted by strip-shaped regions 94B, 94F on the fourth quadrant of the hologram plane 13a are focused on light spots 94BS, 94FS astride the border between detection cells F2d, F1e; and, minus first-order diffracted light beams 104B′, 104F′ (not shown) are focused on light spots 94BS′, 94FS′ formed within the detection cell 7T4.
Since the first-order diffracted light beams 101B, 102B, 103B and 104B are focused on the back side of the photodetective plane 9a (i.e. on the further side from the hologram plane 13a), the light spots formed on the photodetective plane 12a are similar in form to the light distribution on the hologram plane 2a. Since the minus first-order diffracted light beams 101B′, 102B′, 103B′ and 104B′ are focused on the front side of the photodetective plane 12a (i.e. on the side nearer to the hologram plane 13a), the light spots formed on the photodetective plane 12a are similar in form to a light distribution obtained by inverting the light distribution on the hologram plane 13a with respect to the point 100. Since the first-order diffracted light beams 101F, 102F, 103F and 104F are focused on the front side of the photodetective plane 12a, the light spots formed on the photodetective plane 12a are similar in form to a light distribution obtained by inverting the light distribution on the hologram plane 13a with respect to the point 100. Moreover, since the minus first-order diffracted light beams 101F′, 102F′, 103F′ and 104F′ are focused on the back side of the photodetective plane 12a, the light spots formed on the photodetective plane 12a are similar in form to the light distribution on the hologram plane 13a. 
Here, the first optical disc 6 is a DVD, and the second optical disc 6′ is a CD, for example.
Some of the detection cells are connected electrically, and as a result, the following six signals can be obtained.
F1=(a signal obtained in the detection cell F1a)+(a signal obtained in the detection cell F1b)+(a signal obtained in the detection cell F1c)+(a signal obtained in the detection cell F1d)+(a signal obtained in the detection cell F1e)                F2=(a signal obtained in the detection cell F2a)+(a signal obtained in the detection cell F2b)+(a signal obtained in the detection cell F2c)+(a signal obtained in the detection cell F2d)+(a signal obtained in the detection cell F2e)        T1=(a signal obtained in the detection cell 7T1)        T2=(a signal obtained in the detection cell 7T2)        T3=(a signal obtained in the detection cell 7T3)        T4=(a signal obtained in the detection cell 7T4)        
In FIGS. 9 and 10, with the Y-axis indicating the radial direction of the optical disc (disc-radial direction), a focus error signal FE onto the signal plane of the optical disc, a tracking error signal TE onto a track of the optical disc, and a reproduction signal RF for the signal plane of the optical disc are calculated based on the following formulae (1)-(3).FE=F1−F2  Formula (1)TE=T1+T4−(T2+T3)  Formula (2)RF=F1+F2+T1+T2+T3+T4  Formula (3)
In FIG. 9, D1 denotes the distance from a virtual emission point 90 of the first laser beam to each of the boundary between the light spots 82FS and 82BS, the boundary between the light spots 83FS and 83BS, the boundary between the light spots 82FS′ and 82BS′, and the boundary between the light spots 83FS′ and 83BS′. (D1+D1′) denotes the distance from the virtual emission point 90 of the first laser beam to each of the boundary between the light spots 81FS and 81BS, the boundary between the light spots 84FS and 84BS, the boundary between the light spots 81FS′ and 81BS′, and the boundary between the light spots 84FS′ and 84BS′. Similarly in FIG. 10, D2 denotes the distance from a virtual emission point 90′ of the second laser beam to each of the boundary between the light spots 92FS and 92BS, the boundary between the light spots 93FS and 93BS, the boundary between the light spots 92FS′ and 92BS′, and the boundary between the light spots 93FS′ and 93BS′. (D2+D2′) denotes the distance from the virtual emission point 90′ of the second laser beam to each of the boundary between the light spots 91FS and 91BS, the boundary between the light spots 94FS and 94BS, the boundary between the light spots 91FS′ and 91BS′, and the boundary between the light spots 94FS′ and 94BS′. Since the distance from the point 90 and the point 90′ as the virtual emission points of the laser beam are approximately proportional to a diffraction angle, and since the diffraction angle is approximately proportional to a wavelength, the following Formula (4) is established.D2/D1=D2′/D1′=λ2/λ1  Formula (4)
Here, the conventional photodetective pattern has a shape extending in the Y-axis direction, and thus, even if the wavelength varies, the light spots 81FS′ and 81BS′, and the light spots 91FS′ and 91BS′ enter the detection cell 7T1; the light spots 82FS′ and 82BS′, and the light spots 92FS′ and 92BS′ enter the detection cell 7T2; the light spots 83FS′ and 83BS′, and the light spots 93FS′ and 93BS′ enter the detection cell 7T3; and, the light spots 84FS′ and 84BS′, and the light spots 94FS′ and 94BS′ enter the detection cell 7T4. With respect to each of the laser beams, a tracking error signal TE can be obtained in the same manner. On the other hand, the light spots 81FS and 81BS, 82FS and 82BS, 83FS and 83BS, 84FS and 84BS, light spots 91FS and 91BS, 92FS and 92BS, 93FS and 93BS, 94FS and 94BS are shaped to extend less in the X-axis direction and arranged substantially along the Y-axis, and thus, even when the wavelength varies, these light spots just shift along the Y-axis. Therefore, from the detection cells F1a, F2a, F1b, F2b, F1c, F2c, F1d, F2d, F1e and F2e, a focus error signal EF can be obtained corresponding to any of the laser beams.
As mentioned above, from the conventional photodetective pattern, desired focus error signal FE, tracking error signal TE, and reproduction signal RF can be obtained respectively with respect to two laser beams.
Patent document 1: JP 2000-132848 A